1. Field of the Invention:
This invention relates to active type auto-focus cameras, and more particularly to auto-focus cameras in which the light transmitted toward the object to be photographed and received by the focusing system to focus on the object has a wavelength outside the region of the wavelengths that form the photographic image.
2. Description of the Prior Art
Prior art cameras may focus by projecting light from a light-emitting element in the camera onto an object to be photographed. They may automatically measure the degree of sharpness of images in television cameras, video cameras, or still cameras. When such cameras use visible light as the range-finding beam they have the disadvantage of forming an image of the visible-light-emitting element on the film plane.
Several such active focus detecting methods have been proposed using a region of wavelengths, such as infrared light to which the film is insensitive.
However, the use of infrared light as a range finding beam sometimes lowers the accuracy of the focus detection because chromatic aberrations of the objective lens produces a difference in the in-focus positions as measured by infrared light and that suitable for photographic light. This is especially so when detecting the focus by projecting the infrared light through the objective lens onto the object to carry out the in-focus detection. The influence of the chromatic aberration of the objective lens is then no longer negligible.
The principles of this TTL active system are depicted in FIGS. 1(a) to 1(c). In FIG. 1(a), a projection lens system 2 for projecting light from a light source 1 has an optical axis 01 inclined to an optical axis 0 of an objective lens 3, and also an optical axis 02 of a light receiving lens system 4 inclined thereto. A light bundle L1 emanating from the light source 1 after having been focused primarily on a plane F conjugate to the film surface by the projection lens system 2, is projected by the objective lens 3 on an object to be photographed S. Then, the reflected light L2 from the object S is focused by the objective lens 3 to form a primary image at or near the plane F and then by the light receiving lens system 4 to form a secondary image on a sensor 5.
With such an arrangement, when the objective lens 3 is in-focus on the object S, as illustrated in FIG. 1(b), the projected light L1 hits the object S at its center, and, therefore, the light reflected through the objective lens 3 and collection lens system 4 forms the secondary image at a central portion of the image receiving surface of the sensor 5. On the other hand, when out of focus, the target point of the projection light L1 on the object S is shifted upward or downward from the center, with the result that the secondary image with the reflected light L2 by the objective lens 3 and the collection lens system 4 changes its position on the image receiving surface of the sensor 5 depending upon the respective condition of the near or far focus as illustrated in FIGS. 1(a) and 1(c) respectively. Therefore, the use of a line sensor consisting of two photo-sensitive elements as the sensor 5 provides the possibility of discriminating between each condition of focus, e.g. the in-focus, near focus and far focus conditions, when the positions of the secondary images on the respective sensor elements relative to each other is detected.
The above-described TTL active method is, thanks to the involvement of the objective lens 3 in the range-finding process, free from parallax, and also allows the objective lens to be interchanged. It further assures accurate and reliable control of the operation even when the brightness of the object S is low.
However, such a statement appears to be valid provided that the wavelength of the range-finding light beam L1 does not deviate far from the main region of wavelengths contributing to the photograph. Otherwise, for example, when the wavelength of the projection light L1 lies in the infrared region, since the objective lens 3 generally has chromatic aberrations, a discrepancy will arise between the detected and actual focusing positions. Even in such a case, if that discrepancy has a specific value, it is necessary only to set up an optical arrangement that shifts the primary focal plane F' for the range-finding light beams L1 and L2 at the time of establishment of the in-focus condition by a distance, .alpha., with respect to the conjugate plane F to the film surface, as shown in FIG. 2. In single lens reflex cameras where as the objective lens can be interchanged, and the deviation varies depending on .alpha., the characteristic of the lens used, such a method has been found to be incapable of affording sufficient correction.
FIG. 3 shows the relation between the amount of chromatic aberration .DELTA. of light in the infrared region with respect to the wavelength (for example, Fraunhofer's d-line) contributing mainly to photography of an objective lens and the focal length f of the objective lens. In FIG. 3, the solid line represents focusing of the lens with the object at infinity, and the dashed line represents it with the object at a close distance. Strictly speaking, the amount of chromatic aberrations is changed lens by lens according to the tendency to chromatic aberration, active type, zoom type, or the method of evaluating the chromatic aberration. Hence all lenses do not always fall on the curves of FIG. 3. But generally speaking, the longer the focal length and the shorter the object distance, the larger the amount of chromatic aberrations tend to become. This is common to many photographic objectives. As is evident from this graph of FIG. 3, in order to allow all of these photographic lenses to be used as the objective lens of the TTL active system, despite a certain depth of focus is taken into account, some correction must be made.